Structural characteristics and pyrolysis behavior of low-rank coal with different vitrinite/inertinite ratio

Pyrolysis is the basis of clean coal technologies such as coal liquefaction and gasification, especially for the low-rank coal. To further investigate the pyrolysis characteristics of low-rank coals, a series of different vitrinite/inertinite ratio ZJZ, MTH, and DT coals (V/I: 0.1, 0.76, and 1.76 respectively) are used as the research objects. The chemical structure of the sample was analyzed with Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction technology (XRD). The pyrolysis process and gas release behavior of coal samples at 30−900 °C was checked by TG-MS. The results show that the chemical structure content of low-rank coals with different V/I is quite different. Compared with the inertinite-rich ZJZ coals, the vitrinite-rich coals have relatively rich aliphatic structures, long aliphatic chains, and oxygen functional groups. Especially for DT coal, it has more C—O content. The corresponding aromatic structure content decreases with the increase of the mirror-inert ratio. With the increase of V/I in the coal, the aromatic layer spacing d₀₀₂, the ratio of the aromatic layer size to the aromatic layer stacking height (L_a/L_c) increase, andL_c,L_a and the number of stacking layers (N) decrease. The weight loss ratio of DT, MTH, and ZJZ coal during pyrolysis is 36.4%, 32.2%, and 28.9% respectively. As theV/Idecreases, the final pyrolysis gaseous product yield and the maximum pyrolysis rate decrease accordingly. During the pyrolysis process, the small molecular gas, such as H₂, H₂O, CH₄, CO, CO₂, are released; In addition, the content of released gas is closely related to the maceral composition. The mass of small molecules produced by inertinite-rich coal is almost lower than that produced by vitrinite-rich coal. Because the vitrinite in coal contains more aliphatic structures, and the polycondensation capacity between the aromatic layers is stronger, the free radicals fragment are easily formed during the thermal process and then combined into gas.